It may have been mentioned in this thread before, but I did not see it...
I currently have a half helmet, but I am not sure I am comfortable with it for safety reasons. I wore nothing but full face when I rode sport bikes, but I remember how hot those were at times. And hard to sip a drink from, and hard to kiss the girl with, and so on and so on, unless you took the helmet off. Even having a conversation at a light was tough over engine noise and yelling into a flat surface an inch in front of your face.
So I looked for options. After looking at a lot of the convertible helmets I have become real interested in the Shark Evoline 2. The thing is way too expensive, and heavy, but the reviews seem to be good. People are saying the weight is not noticeable when on, and balances well when in the converted mode. Basic idea is that is goes from full face to three quarters by moving the chin bar up and behind the head. It is one of the few I found that is fully DOT approved even with the chin bar locked back. Most say to only flip the top up at a full stop, and to make sure it is locked down if the bike is moving. With the Shark you can ride in either mode and be assured that at least it was tested and approved.
Now I just need to find a place that actually carries them around Denver. I am not going to order one on-line and deal with it not being the right shape, getting the wrong size, and the hundred other issues that would cause me to have to ship it back and forth until I was happy with the feel.
So I can not endorse it directly, but it sure piqued my interest. Lots of good vids on it, so do a search and take a look if you think it might work for you.